1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a semi-transmission type liquid crystal display device capable of ensuring a large viewing-angle, improved productivity, suppressed power consumption and increased yield by optimizing the design of its optical elements.
2. Description of the Prior Art
Liquid crystal display devices are generally classified into transmission-type devices and reflection-type devices. The transmission-type devices utilize the light irradiated from their backlights for display. They are widely used as the display devices of, e.g., word processors, laptop personal computers. However, they cannot provide proper display if they are used in an environment (e.g., outdoor) having a large intensity of incident light on them.
The reflection-type devices reflect external light and use it for display. They consume less power than the transmission-type devices because they do not carry backlights. Accordingly, they are drawing attentions as display devices of portable appliances, which are becoming popular rapidly.
Such a reflection-type liquid crystal display device comprises a lower substrate, a reflection electrode, a lower orientation film, a liquid crystal layer, an upper orientation film, an upper transparent electrode, a color filter, an upper substrate, an optical compensation film, and a polarization plate. The upper and lower substrates are spaced a determined distance and a liquid crystal is injected between them. The phase of the liquid crystal used in the reflection-type liquid crystal display element includes a nematic phase and a cholestric phase. When the nematic phase is used, the liquid crystal has a molecular array wherein all of the liquid crystal molecules are arranged substantially homogeneous or homeotropic relative to the surfaces of the upper and lower substrates. The array has a twisted azimuth wherein the molecules are twisted a determined angle continuously.
The display implementation of the reflection-type liquid crystal display device according to its optical characteristics will now be explained.
When there is no voltage applied, linearly polarized light from the polarization plate is changed into circularly polarized light, e.g., leftward circularly polarized light, after passing through a phase film. The light is again changed into linearly polarized light, after passing through the liquid crystal layer, and then reflected by a reflection plate. As the reflected light passes through the liquid crystal layer again, it is changed into leftward circularly polarized light. After passing through the phase film, the light is transformed into light whose polarization direction is parallel to the polarization axis of the polarization plate. If the transformed light passes through the polarization plate, a white state is implemented.
When a voltage is applied, the leftward circularly polarized light from the polarization plate and the phase film passes through the liquid crystal layer without any change. Then, the light is reflected by the reflection plate and is changed into rightward circularly polarized light. As the light passes through the liquid crystal layer and the phase film again, it is transformed into linearly polarized light whose polarization direction is perpendicular to the polarization axis of the polarization plate. The transformed light cannot pass through the polarization plate and, consequently, a dark state is implemented.
In the case of the reflection-type liquid crystal display device, good display screen depends on how the characteristic values of each of the components are optimized. More specifically, in order to effectively improve the reflectivity of the reflection-type liquid crystal display device, the transmission axis angle of the polarization plate, the optical characteristics of the phase film, the width d of the liquid crystal layer, the phase delay value dΔn of the liquid crystal layer, the twist angle of the liquid crystal, the orientation angle of the orientation film, and the characteristics of the reflection plate should be optimized.
FIG. 1 is a sectional view illustrating a reflection-type liquid crystal display device according to the prior art.
The reflection-type device, as shown in FIG. 1, has a structure comprising a lower substrate 10 with a reflection plate 11 and a lower orientation film 12, an upper substrate 13 with a color filter 14 and an upper orientation film 15, a liquid crystal layer 16 interposed between the lower and upper substrates 10, 13, as well as a λ/4 film 17 and a polarization plate 18 positioned on the outer surface of the upper substrate 13 in series. The λ/4 film 17 is a mono-axial film for the optical compensation of light having a phase differential of λ/4.
The reflection-type liquid crystal display device according to the prior art, having a structure as mentioned above, has a twist angle of 90° and the angle between the optical axis of the λ/4 film 17 and the axis of the polarization plate 18 is 45°.
However, the reflection-type liquid crystal display device according to the prior art has a problem of poor display characteristics, because its reflectivity is decreased if two λ/4 films are used to optimize its cell design and it cannot properly perform the function of providing a phase differential of λ/4 in the broad band of visible ray wavelength if one λ/4 film is used to avoid the reflectivity decrease.
Furthermore, the reflection-type device according to the prior art has very small cell gap and therefore has poor yield when applied to actual process.